Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. One method of variable cam timing uses an Oil Pressure Actuated device, such as a vane type cam phaser. The phaser may be controlled by an electromechanically actuated spool valve that directs oil flow to one side or the other of the vane. The performance of this device is thus dependent on oil pressure, which can be a function of engine speed and leakage through various oil systems.
One example VCT system uses a spool valve with one inlet and two outlet flows. The spool valve thus has two functions—to let control oil in and to let drain oil out. The control oil inlet is typically used to control VCT position, often through feedback and/or feed forward methods.
The inventors herein have recognized that at high engine RPM, available control systems may be insufficient to accurately control VCT position during cam torque oscillation events while maintaining cam timing at a current cam timing. Significant amounts of oil may be unintentionally drained out of the system, necessitating additional control oil to be taken in during future control events. Additionally, when oil is forced out of the drain ports, the cam may move out of its desired position, or may oscillate to a greater degree. This may result in both undesired engine knocking and subsequent degradation to engine parts.
In one example, these issues may be addressed by a method, comprising actuating a spool valve via a solenoid to advance and retard a hydraulic variable cam timing actuator coupled to a camshaft; and, while commanded to a null position to maintain current cam timing, modulating the solenoid with a component at a frequency synchronous to engine firing frequency to counteract cam torque oscillations.In another example these issues may be addressed by a system for an engine, comprising a hydraulically actuated variable cam timing actuator, a cam with oil passages used to deliver oil from oil passages in a cam cap to the variable cam timing actuator coupled to the cam, a hydraulic spool valve coupled to the variable cam timing actuator, the spool valve adjustable among a plurality of positions including a null position to maintain current cam timing, and a solenoid configured to control the position of the spool valve by modulating the spool valve in the null position with a component at a frequency synchronous to engine firing frequency to counteract cam torque oscillations.
In this way, it is possible to generate actuation of the spool valve to counteract the effects of the cam torque on the oil flow, so that the oil volume in the variable cam timing actuator is maintained and the cams better maintain the current cam timing. Specifically, while commanded to the null position to maintain current cam timing, for example based on feedback and/or feedforward control, the solenoid modulation enables the spool valve to block the flow of oil that may otherwise occur due to the force on the camshaft by the actuation of the cylinder valves. By blocking the unintended flow of oil into, or out of, the hydraulic variable cam timing actuator's advance and retard chambers, the hydraulic system is stiffened and the cams move away from the current timing to a lesser degree, thereby better maintaining the combustion performance consistency across the various cylinders of the engine. Such is possible even with time varying disturbance torques acting on the camshaft from the cylinder valve actuation. Further, by modulating at a frequency synchronous with combustion frequency, it is possible to better align the modulation to the actual kinematic torsion acting on the camshaft and thus counteract and cancel such effects.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.